7.17 Distribution Sampling Technique, Chlorine Residual Monitoring & Nitrification Control in Chloraminated Systems

Sampling the system people actually drink from

WPI lists "collect regulatory and informational water samples," "perform analyses to determine chlorine residual and pH," and "interpret laboratory analysis for chlorine residual, chlorine demand, microbiological results, compliance with established water quality standards, and meeting standard operating practices" as distribution job tasks. The sample siting plan and the Revised Total Coliform Rule are covered in the Regulation 11 section; this section is about how the distribution operator executes the program in the field.

The sample siting plan

Every public water system operates under a written sample siting plan approved by CDPHE. It identifies routine sampling sites, repeat sites upstream and downstream of each routine site, and the schedule. Sites must be representative of water throughout the distribution system, which in practice means:

• Distributed across pressure zones, not clustered near the plant.
• Including areas of low flow and long detention, such as dead ends and system extremities, because that is where residual is lost and coliform appears.
• Including sites downstream of storage facilities.
• Avoiding sites where the tap itself is the problem.

Sampling location matters as much as the schedule. The plan is not a suggestion, and taking a sample from a convenient tap instead of the designated one invalidates it.

Collecting a bacteriological sample correctly

The single most common cause of a false total coliform positive is sampling technique, and the consequences — repeat samples, assessments, possible public notice — are expensive.

1. Select a cold water tap directly on the service, free of aerators, screens, swivel spouts, hoses, and water treatment devices.
2. Remove any attachment. Do not sample from a hose bib where the plan calls for an interior tap, and never from a frost-proof yard hydrant unless approved.
3. Flush long enough to draw water from the main, not from the building plumbing — commonly 3 to 5 minutes, and until temperature stabilizes.
4. Adjust to a smooth, moderate flow without splashing.
5. Use the sterile bottle containing sodium thiosulfate, which neutralizes the chlorine residual so bacteria are not inactivated between collection and analysis.
6. Do not rinse the bottle, do not touch the inside of the bottle or cap, and do not set the cap down.
7. Fill to the 100 mL line, leaving air space so the laboratory can shake the sample.
8. Record site, date, time, collector, and the free and total chlorine residual and pH at the moment of collection.
9. Complete the chain of custody, ice the sample, and deliver within the holding time — 30 hours for compliance coliform samples.

Measure and record the disinfectant residual with every bacteriological sample. Under the Revised Total Coliform Rule, residual and coliform results are read together: a coliform positive at a site with no measurable residual points to a distribution problem, while a coliform positive at a site holding a good residual more often points to the tap.

Chlorine residual monitoring

• DPD colorimetric is the standard field method. The first reagent gives free chlorine; adding potassium iodide gives total chlorine. Total minus free equals combined chlorine (chloramines).
• Amperometric titration is the reference method for low-level and effluent work.
• Continuous online analyzers at entry points and remote sites feed SCADA and trigger alarms.
• Chlorine demand equals dose minus residual. A rising demand at a fixed dose means something changed: more organics, nitrification, biofilm growth, or a new source in service.

Nitrification in chloraminated systems

This is the distribution water quality problem most likely to appear as a scenario question, and Regulation 100 recognizes it by raising a system to Class 2 when chloramines are used instead of free chlorine.

Chloramine is formed from chlorine and ammonia. Excess free ammonia — from overfeeding ammonia, from an incorrect chlorine-to-ammonia ratio, or from chloramine decay in long-detention water — becomes food for ammonia-oxidizing bacteria. Those bacteria convert ammonia to nitrite, and nitrite exerts a very large chlorine demand, which destroys more chloramine, which releases more ammonia. It is a self-accelerating loop.

The signature of nitrification:

Nitrification is worst in warm water, in long-detention areas — storage tanks, dead ends, and oversized mains — and where the chlorine-to-ammonia-nitrogen ratio has drifted from the target of roughly 4:1 to 5:1 by weight.

Responses, roughly in order:

1. Increase turnover in tanks and flush dead ends to remove the aged water.
2. Correct the chlorine-to-ammonia ratio so free ammonia is minimized.
3. Boost the chloramine residual in the affected area.
4. Perform a temporary free chlorine conversion — a "chlorine burn" — running free chlorine through the system for several weeks to knock down the nitrifying population. This must be planned, because free chlorine mobilizes different byproducts and taste and odor and requires customer notice.
5. Clean or replace tanks and mains with heavy biofilm and sediment.

Monitoring nitrite routinely at representative sites is how a chloraminated system catches nitrification early rather than discovering it as a residual collapse.